A woman can get pregnant at any time in her menstrual cycle, however the most fertile phase is about two weeks after her period. It is most important to follow safe birth control practices at all times, unless pregnancy is intended.
In a study conducted by the United States National Institutes of Health, a woman could get pregnant anytime. I have personally assisted two births with mothers practicing the rhythm method (perfect practice). If a woman's cycle is for 28 days (regular), the "rainy days" are days 10 to 14 with day 1 as the first onset of the period. This is the time that it is likely that a woman would get pregnant.